A potential tipping point in tropical agriculture: Avoiding rapid increases in nitrous oxide fluxes from agricultural intensification in Kenya

Abstract, Biogeosciences, 2015

There are national and regional efforts aimed at increasing fertilizer use in sub‐Saharan Africa, where nitrogen (N) inputs must be increased by an order of magnitude or more to reach recommended rates. Fertilizer inputs increase N availability and cycling rates and subsequently emissions of nitrous oxide (N₂O), a powerful greenhouse gas and the primary catalyst of stratospheric ozone depletion. We established experimental maize (Zea mays L.) plots in western Kenya to quantify the relationship between N inputs and N₂O emissions. Mean N₂O emissions were marginally, but not significantly, better described by an exponential model relating emissions to N input rate in 2011; in 2012, an exponential relationship provided the best fit compared to linear and other nonlinear models. Most N₂O fluxes occurred during the 30 days following the second fertilizer application. Estimates of fertilizer N lost as N₂O annually were well below the 1% Intergovernmental Panel on Climate Change default emission factor, ranging from 0.07% to 0.11% in 2011 and from 0.01% to 0.09% in 2012. In both years, the largest impact on annual N₂O emissions occurred when inputs increased from 100 to 150 kg N ha⁻¹: fluxes increased from 203 to 294 g N₂O‐N ha⁻¹ yr⁻¹ in 2011 and from 168 to 254 kg N ha⁻¹ in 2012. Our results suggest that exponential emission responses are present in tropical systems and that agricultural intensification in western Kenya may be managed for increasing crop yields without immediate large increases in N₂O emissions if application rates remain at or below 100 kg N ha⁻¹.